Numerous hydraulic actuators are known which embody air-piloted directional control valves with accumulators used as auxiliary pressure sources to supplement hydraulic pump discharge. However, known hydraulic actuators of this character are not suited for actuating, for example, the main steam isolation valve or a feed water valve or a coolant valve in an electric power generator plant which requires a continuous duty actuator. Such valves are generally large size gate valves which require a hydraulic actuator cylinder having an active stroke, of say 24", and, moreover, the hydraulic actuator must be able to close the open valve within a short period of time e.g. 5 seconds or less especially in the event of an accident condition such as a steam line break. A further requirement of such valve actuator is that it be fail-safe with the valve being quickly closed in the event of air and/or electrical power failure. Still another requirement of such actuator, is that it be provided with dual (active and standby) hydraulic and pilot air circuits to enable operation of the actuator to open or close the valve by using either circuit when the other circuit is inoperative or requires servicing.
Still further a valve of the character indicated is intermittently operated relatively infrequently and hence provision must be made for periodic checking of the pre-charge of the active and standby circuit accumulators.